Though there's currently no word on whether it will fall into the "New Super Mario Bros." series.
Though only a minor mention in Nintendo's third quarter investor briefing, it seems yet another 2D side-scrolling Mario game is in the works for the 3DS. According to Nintendo, the two Mario games released in 2011 for the 3DS - Super Mario 3D Land and Mario Kart 7 - are their centerpiece evergreen titles. That information combined with the overwhelming success of both New Super Mario Bros. and New Super Mario Bros. Wii make a new 2D side-scrolling Mario game an easy call.